As shown in FIG. 1, a Coordinate Measuring Machine, or CMM, comprises a measuring apparatus 2, a controller 10, and a computer 12. The measuring apparatus 2 includes a table 4 that generally extends in an XY plane, a bridge 6 spanning the table in the X direction and a carriage 8 supported on the bridge 6. The carriage 8 supports a Z-ram 9, which can move vertically, or in the Z direction. At the end of the Z-ram 9 is an articulating probe 15, such as the type disclosed in the above-referenced U.S. Pat. Nos. 7,213,344; 5,665,896 and 4,888,877.
As is well known, the computer 12 and controller 10 cooperate to drive motors that move the carriage 8, bridge 6 and Z-ram 9 for the purpose of measuring work pieces situated on the table 4. Part of the articulating probe 15, generally a stylus, contacts the work piece and includes switches or sensors that trigger the computer 12 to take a measurement.
The foregoing describes a direct-control CMM, meaning that the CMM can be controlled directly from the computer 12, but direct-controls are but one type of CMM available on the market. Other CMMs are manually controlled or are otherwise not directly controlled by the computer 12. As is well known, manual CMMs have different configurations because of the lack of drive motors.
CMMs are calibrated in the factory using well-known techniques to generate an error map that compensates for errors, including errors introduced during the manufacturing of the CMM. CMM manufacturers typically check, or validate, the quality of the error map itself as part of the process to ensure optimum performance of the CMM. To validate an error map a technician places an artifact, such as a standard ball bar 100 of known length, on the table 4 and brings the probe, whether articulating or not, into contact with the balls on the end of the ball bar as shown in FIG. 2. The spheres may be contacted in more than one position in order to determine the center of this sphere. The measured length of the ball bar is calculated to be the distance between the measured centers of the balls. This can be compared to the known length of the ball bar to validate the error map of the CMM. Generally, the ball bar is measured in several different orientations and locations in the CMM's measuring volume.
One of ways to validate an error map is to validate the squareness error between two of the CMM's axes. In a typical process for validating a squareness error map, a technician typically places an artifact, such as the ball bar 100, on the table 4 of the CMM in two different orientations at two different times. In one orientation, a radial orientation, shown schematically in FIG. 3, the ball bar 100 is oriented to form a 45° angle with the X-axis, while in another orientation, a tangential orientation, the ball bar 100 is oriented at 135 degrees with respect to the x-axis. (Incidentally, while FIG. 3 does not indicate the precise location of the ball bar 100 on the table 4, a technician of ordinary skill knows where to position the ball bar on the table to correctly validate squareness error.)
The squareness error is ten approximated by the following equation:S=(LRAD−LTAN)/LNOM,
Where LRAD is the length of the ball bar in the radial position, LTAN is the length of the ball bar in the tangential direction and LNOM is the known length of the ball bar. If the squareness error is larger than a specified quantity, then the technician knows that the CMM's error map needs correcting.
However, the process of moving ball bars or other calibration artifacts around in the measuring envelope of the CMM is time consuming, and therefore costly.